Coordinate measuring machines, generally referred to as CMM's, are by now a well known means of checking the accuracy of manufactured parts, such as stamped sheet metal body panels for automobiles. With CMM's, some kind of fixture is needed to hold the part to be checked in a known orientation, one which presents the part in a fixed reference frame accurate in all three axes. Ideally, the fixture would hold the part with sufficient accuracy that the part could be read immediately. That is, if the fixture were accurate enough per se, it would not be necessary to take initial readings on the part to establish the origin of the part. However, that level of accuracy is not available with some current systems.
One option is the so called "hard" or dedicated fixtures. A dedicated fixture is a large, solid, heavy part that is unique to the panel to be measured, just as the die that makes the panel is unique to the panel. Although accurate, a hard fixture is expensive to make and store. More troublesome than initial cost, in the current climate of flexible manufacturing and the imperative toward ever more rapid die changes, is the time involved just in finding and setting up a dedicated fixture.
So called universal fixtures accommodate several different parts. A heavy foundation or base has a peg board grid of drilled holes in which various clamps or holders can be removably mounted. Part holders are placed at as many locations on the base as necessary to support any particular part. The base is fixed accurately relative to the CMM and, if the part holders are also fixed accurately relative to the base, the net effect is that the part is fixed accurately relative to the CMM. Not only is it critical that the part holders be secured accurately relative to the base, it is also important that the part holders be capable of quick and easy set up on and removal from the base, so that the different parts to be measured can be fixtured quickly.
Known universal fixtures fall short of these goals. One well known universal fixtures uses a number of vacuum cups threaded into drilled holes in a plate. These pull down on the underside of the part to support it. A vacuum suction cup, being flat and flexible in the vertical direction, cannot fix an absolutely accurate lateral or vertical position of the supported part, as is described in more detail below. As a consequence, it is recommended that parts fixtured by the vacuum type of device be initially read to establish part origin, rather than trusting the fixture alone for accuracy. Other universal fixtures use the same type of base with drilled threaded holes, but use purely mechanical clamps and supports, avoiding the inevitable inaccuracies inherent in the flexible vacuum cup system. Such clamps are secured to the base by multiple threaded bolts. Changing the part to be measured generally requires that each clamp be removed and replaced with another, in turn requiring that each bolt be removed and replaced with another in a different location. This is obviously slower than just unthreading and moving a single vacuum cup.